It is imperative in stereotactic radiation treatment to ensure that radiation beam, such as a linear accelerator (LINAC) central beam of radiation, passes as close as possible to the ideal isocenter, i.e., the theoretical intersection of a rotational axis (azimuth axis) of a table which carries the target (patient) and a rotational axis (elevation or roll axis) of the LINAC gantry. In general, the target lies in the patient's head and a stereotactic frame is provided for holding the head fixed with respect to the floor or the treatment table. The purpose of fixing the stereotactic frame is to ensure that the radiation beam always passes substantially through the isocenter during radiation treatment. One way of fixing the stereotactic frame is by mounting it on the table. Another way is to fix the stereotactic frame with respect to the floor. In each case, means are provided for attempting to position the frame such that the target is substantially in the isocenter.
In practice, however, while the heavy gantry moves to various gantry angles, the LINAC head sags. Moreover, the rotary bearing of the LINAC and/or the table are not ideal and this affects concentricity, inter alia. Consequently, the central radiation beam does not intersect the ideal isocenter. Thus, in general, unpredictable deformations of the structures supporting the radiation source or the target, as well as errors in determining the positions of the source or the target lead to inaccuracies in delivering the radiation beam to the isocenter.